Heat capacities of two metal-assembled complexes {NBu4[(CuCrIII)-Cr-II(ox)(3)]}(n) and {PPh4[(MnCrIII)-Cr-II(ox)(3)]}(n) (Bu=n-butyl, Ph=phenyl, H(2)ox=oxalic acid) were measured by adiabatic calorimetry in the 0.5-300 K temperature range. A ferromagnetic phase transition was detected at T-c=6.98 K for {NBu4[CuCr(ox)(3)]}(n) and T-c=5.59 K for {PPh4[MnCr(ox)(3)]}(n), above which a remarkable heat capacity tail suggesting the short-range order effects was observed. Furthermore, a lambda-type heat-capacity anomaly due to a structural phase transition was found at T-trs=226.9 K for {NBu4[CuCr(ox)(3)]}(n) and at T-trs=71.3 K for {PPh4[MnCr(ox)(3)]}(n). The observed entropy gains due to the magnetic phase transitions are very close to the theoretical values, R ln(2x4) for {NBu4[CuCr(ox)(3)]}(n) and R ln(6x4) for {PPh4[MnCr(ox)(3)]}(n), expected from the spin multiplicities (Cu-II, s=1/2; Mn-II, s=5/2; Cr-III, s=3/2). Since this series of metal oxalato assemblies can crystallize in either 2D honeycomb or 3D helical hetero-spin lattices, the theoretical magnetic heat capacities for both lattices were calculated by the high-temperature series expansion up to seventh cumulant to compare with their experimental magnetic heat capacities. The magnetic heat capacities above T-c were reproduced well by the theoretical ones for the 2D honeycomb lattice rather than the 3D helical lattice. The intralayer exchange interaction was estimated to be J/k(B)=5.0 K for {NBu4[CuCr(ox)(3)]}(n) and J/k(B)=0.95 K for {PPh4[MnCr(ox)(3)]}(n). The analyses based on spin wave theory revealed that both compounds bring about dimensional crossovers into 3D ferromagnetic orders below T-c through the weak interlayer interactions. (C) 2003 American Institute of Physics.